Preliminary studies of aqueous extracts of rat testes and ram rete testis fluid demonstrate a dose dependent suppression of FSH and LH in an in-vitro bioassay utilizing LRF stimulated gonadotropin release from neonatal rat pituitary explants. This gonadotropin suppression does not appear to be due to sex steroid contamination. These data are encouraging for the hypothesis that a non-steroid FSH inhibitor (inhibin) is produced by the seminiferous tubule.